1. Field of the Invention
The present invention relates generally to a method and apparatus for measuring the luminous intensity of incident light and, more particularly, to a technique for correcting the light intensity as measured by a light amount measurement device using a light chopper plate, for example.
2. Related Art
For measurement of the intensity of an incoming beam of signal light, a photosensitive element serving as a photodetector is used to measure the intensity of signal light received thereby. Here, currently available photosensitive elements have relatively large temperature dependency and thus are faced with a problem as to deficiency of sensitivity stability. In particular, for high-sensitivity photodetective elements operable to measure an extra-low intensity of light, this instability problem occurs severely and is not negligible.
A prior known approach to avoiding this problem is to split or “diverge” a portion of signal light by a beam sampler for causing a diverged light component to be coupled with an optical fiber. Then, attenuate the diverged light to a level equivalent to that of the signal light containing therein optical measurement information, which light is then introduced into a photoelectron multiplier tube. Next, a pendulum-like swingable rotation shutter is used to selectively block or “shield” the diverged light and the signal light, causing the diverged light and signal light to enter the same photoelectron multiplier tube in a time-series manner. Prior art light intensity measuring equipment employs such optical configuration to perform measurement of the signal light intensity while at the same time correcting a measured signal light amount variation by letting the light-path interruption operations of the diverged light and signal light be synchronized with each other. A device using this technique is disclosed, for example, in JP-A-4-262243, which is designed to perform the measured light amount variation correction, also including sensitivity drift correction or else of a photoelectron multiplier tube for use as a photosensitive element, to thereby improve the optical measurement accuracy.
Unfortunately, the prior art configuration does not come without accompanying a penalty as to increased complexity of an optical system which produces the diverged light acting as reference light used for execution of the measured light amount variation correction, including but not limited to a structure for partially diverging the signal light by a beam sampler or a structure for selectively interrupting the signal light and the light which is diverged by a pendulum-like swingable rotation shutter. Another problem faced with the prior art is that luminous intensity measurement of the reference light has no more than an ability to control the distance between optical fiber end faces for coupling the diverged light, resulting in the lack of either the light intensity adjustment setup accuracy or the degree of freedom.
As apparent from the foregoing, prior art light amount measurement devices suffer from the optical-system complexity problem and are under strict requirements for performing control of the pendulum-like swingable rotation shutter. Furthermore, due to the fact that the light obtained by divergence of part of the signal light is used as the reference light for correction of variations in measured intensity of the signal light, the prior art lacks the light amount adjustment setup accuracy and/or the degree of freedom—in particular, it suffers from limits in correctable contents, such as an inability to correct the linearity of a photosensitive element for use as the photodetector.